RNA interference (RNAi) has taken experimental biology by storm in recent years, since it has been greatly exploited as an ever increasingly effective tool to study gene function by gene silencing through RNAi. It is also being developed as a possible therapeutic tool. The RNAi machinery has been shown to have a role in development and appears to be important in maintaining stem cell identity. Though there has been remarkable progress in unraveling the components of the RNAi machinery, we are just beginning to understand how they work at the molecular level. The objective of this proposal is to provide the structural and biochemical framework, which will enable us to understand the mechanism of gene silencing by RNAi, with a focus on the signature component of the RNA-induced silencing complex (RISC), the Argonaute protein.